NAT ISSUE

Here is my problem. I am running Windows 2000 NAT. I have both my interfaces set up and on my public interface I have my address pool set with no reservations. The problem is on some, not all websites. I can ping them, but on all my client machines using NAT, the browser shows page not found. Remember I can get to some websites. However if I give the client machine a public IP it can get to the problem websites with no problems. Ive looked at this for days and I am at a loss. It just started happening a few days ago. Any help would be appreciated

Dale